ChemInform Abstract: Determination of Metal Particle Size of Highly Dispersed Rh, Ir, and Pt Catalysts by Hydrogen Chemisorption and EXAFS.

Abstract

023ChemInform Abstract Hydrogen chemisorption measurements cannot be used directly to determine particle sizes of highly dispersed metals due to the uncertainty in the hydrogen to surface metal (H/M) stoichiometry. Therefore, EXAFS measurements are used to determine the metal particle size and to calibrate the chemisorption measurements. It is concluded that the very high H/M values observed for Pt, Rh and Ir are due tomultiple adsorption of hydrogen on metal surface atoms. Differences in the H/M values are due to different valencies of the three group VIII metals and in agreement with the observed order of stability of the corresponding metal polyhydride complexes. It is suggested that hydrogen spillover and subsurface hydrogen give only minor contributions to the high H/M values.

Pt clusters in BaKL zeolite: Characterization by transmission electron microscopy, hydrogen chemisorption, and X-ray absorption spectroscopy

“…Extrapolation of the correlation between H/Pt and the coordination number determined by EXAFS spectroscopy for Pt supported on 7-A1203, determined by Kip et al [9] for a series of samples characterized by different Pt contents and temperatures of treatment in hydrogen, leads to an estimate for the coordination number of 4 for the Pt in zeolite L. This estimate is in agreement with EXAFS results; it has been used to extend the correlation of Kip et al (fig. 6).…”

“…The total number of chemisorbed H atoms was obtained by extrapolating the linear high-pressure part of the isotherm to zero pressure. Details are as given elsewhere [9].…”

Nanoparticle size evaluation of catalysts by EXAFS: Advantages and limitations

“…Supported metal catalysts, especially those of the platinum group are widely used in fuel cells, as well as catalysts for a variety of chemical synthesis and environmental remediation [2,3]. Their small size makes them difficult to characterize by ordinary techniques like chemisorption [4,5], X-ray diffraction (XRD) or transmission electron microscopy (TEM) [6].…”

Exafs evidence for direct Rh-Tan+ bonding and coverage of the metal particles in a Rh/Ta2O5 SMSI catalyst

“…From table 1 it can be concluded that on the average each rhodium atom o had approximately 7.9 rhodium neighbours at 2.658 A, which is slightly shorter than the Rh-Rh bulk distance (2.687 A). From the Rh-Rh coordination number we estimated [10] that the particles were approximately 17 .A in diameter and contained 73 + 5 rhodium atoms. According to [10], the H/Rh value should be 0.95, which agrees excellently with the measured value, H/Rh --0.93, and demonstrates that the metal particles were in the 'normal' state.…”

“…From the Rh-Rh coordination number we estimated [10] that the particles were approximately 17 .A in diameter and contained 73 + 5 rhodium atoms. According to [10], the H/Rh value should be 0.95, which agrees excellently with the measured value, H/Rh --0.93, and demonstrates that the metal particles were in the 'normal' state. No metal-oxygen contribution from the metal atoms in the metal-support interface, such as reported in [2,9], could be detected.…”